SINUS NODE AUTOMATICITY DURING ATRIAL-FIBRILLATION IN ISOLATED RABBIT HEARTS

Background. It is still unclear what role the sinus node may play in the genesis or perpetuation of atrial fibrillation. Therefore, we studied the electrical activity in different regions of the sinus node during atrial fibrillation. Methods and Results. In Langendorff-perfused rabbit hearts, paroxysms of atrial fibrillation were induced by burst pacing. Standard microelectrode techniques were used to record transmembrane potentials from different regions of the sinus node. We found that during atrial fibrillation, a high degree (5:1) of sinoatrial entrance block was present that protected the pacemaker fibers in the center of the sinus node against the high rate of fibrillatory impulses. As a result, the true pacemaker fibers in the center of the node were activated with only a slightly higher average rate than during sinus rhythm. Spontaneous diastolic depolarization was still present but was modulated by electrotonic depolarizations due to intranodal conduction block of atrial fibrillatory impulses. Incidentally, phase 4 depolarization resulted in the generation of spontaneous action potentials in the sinus node. However, the high activation rate in the sinoatrial border during atrial fibrillation prevented these spontaneous impulses to exit from the sinus node. Because of the minimal degree of sinus node overdrive suppression (9%) and the presence of concealed automaticity during atrial fibrillation, spontaneous termination of atrial fibrillation was promptly followed by resumption of normal sinus rhythm. Conclusions. During atrial fibrillation, sinus automaticity still is present in the center of the sinus node and hardly overdrive suppressed due to a high degree of sinoatrial entrance block.